1. Field of the Invention
This invention relates in general to coldworking of holes, and more particularly, to a method and apparatus for coldworking holes, thereby increasing their fatigue life especially in workpieces under stress. This process is especially necessary in the manufacture of airplane wings and body structures.
2. Description of the Prior Art
In the coldworking process, a mandrel of a diameter larger than a hole is pulled or pushed through the hole to expand the hole radially, displacing the material immediately surrounding the hole beyond the elastic limit of the material. Parts having holes in them which are processed in this manner will exhibit improved fatigue life. Conventionally three basic methods of coldworking holes are known in the industry today.
The first method employs a solid mandrel. The solid mandrel has two different diameters and is employed in conjunction with a puller gun. One diameter is sized to just clear the hole to be coldworked, and the other diameter is dimensioned for coldworking. The two diameters are joined by a tapered section to permit a smooth displacement of the material surrounding the hole. This method requires two operators. One operator pushes the small diameter of the mandrel through the hole, and the other operator engages the mandrel with a puller gun and subsequently pulls the large diameter of the mandrel through the hole. Several disadvantages of this method exist. First, the pressures of the workpiece against the coldworking tool and very high, somewhere between the tensile and bearing yield strength of the material being coldworked. This high pressure can cause galling and tool breakage. Another disadvantage is that an operator is required at each side of the workpiece. This mandrel is partly pushed from one direction through the hole by the first operator and attached to a puller gun by a second operator, after which the mandrel is pulled through. Further, the mandrel now has to be returned to the first operator by circumventing the structure containing the hole.
A second method for coldworking holes employs a split mandrel, such as that shown in U.S. Pat. No. 2,357,123, issued to Maxwell. Although the preferred embodiment of the present invention employs a split mandrel, this older method as taught by Maxwell is much different.
Maxwell teaches a split mandrel assembly consisting of a split mandrel on the outside and a tapered pilot on the inside. The tapered pilot enters the hole first, then the split mandrel in its collapsed position also enters and passes through the hole. The tapered pilot is now drawn into the split mandrel thereby expanding the outside diameter of the split mandrel. Subsequently, a shoulder on the tapered pilot seats against the end of the split mandrel and begins to push the expanded split mandrel through the hole, thereby effecting coldworking. This method requires only one operator and is relatively fast. All of the load of pulling, however, is carried by the tapered pilot. The small diameter of the tapered pilot makes the pilot unable to bear such high loads and the tool may fail prematurely. Also, since the split mandrel segments are in compression, they must be sturdy enough not to collapse. The mandrel, therefore, is split only one time into two halves. This results in holes which are oval in shape and therefore of inferior quality. Additionally, because the pilot is tapered, it is difficult to maintain the accuracy of the outside diameter of the expanded split mandrel. Further, the split mandrel generates ridges in the coldworked hole. These ridges reduce the quality of the coldworking, and impact the subsequent reaming operation making it more difficult to produce a quality finish. Finally, in this method the tapered pilot is pulled into the mandrel under a preloaded condition causing premature wearing of the tapered portion of the pilot and the mating surface of the mandrel.
A third method, and one that is quite prevalent today, is the split sleeve and solid mandrel tool. This method described in the U.S. Pat. No. 3,566,662, issued to Champoux permits a single operator to cold work a hole from one side by first introducing a solid mandrel having a reduced diameter portion. A split sleeve rests on the reduced diameter portion during entry into the hole. Then the split sleeve is retained in the hole by the housing of the puller gun while the larger diameter portion of the mandrel is pulled through the split sleeve. It is easy to see several advantages of this presently preferred method: The use of only one operator, the durability of the tool because the tensile stress occurs in a member with a large sectional area, and any slipping occurs between the sleeve and the mandrel, rather than the mandrel and the hole. However, non-symmetrical coldworking is inherent in this method because the sleeve is split only on the side. Also, the operator must install a new sleeve each time a hole is worked, thereby losing time. Also, the sleeves are expensive and must be removed from the hole and discarded.
Another possible relevant patent is U.S. Pat. No. 2,974,712 issued to Frye et al. Frye et al's invention describes a system that would not work for coldworking holes for the purpose of improving fatigue rated assemblies in aircraft. Coldworking holes requires accuracies in the vicinity of one ten-thousandth of an inch. Swage hose fittings, such as what Frye et al's invention is directed to, require accuracy of only two to five one-thousandths of an inch. Frye's conical "ram" or pilot enlarges or expands the mandrel well beyond its free state. Frye's mandrel, after being significantly enlarged during each process, would be required to be frequently tested in insure that the exacting dimensions required were still being met. Further, the relative axial position of the conical pilot and the mandrel changes the outer diameter of the mandrel. If this system were used in coldworking holes, a slight variation in the axial position of the conical pilot and the mandrel would result in a larger or smaller diameter of the mandrel. A change in the mandrel's diameter from one operation to the next operation would be cause for rejection of the tool. The relative position of the conical pilot and the mandrel of Frye may be changed very easily due to small debris, high forces, or being out of proper adjustment. The cylindrical pilot and mandrel of the present invention, however, is not sensitive to relative axial position. Successive coldworked holes will then be exactly the same diameter even if the pilot is in a slightly different axial position within the mandrel. In other words, to operate properly, the pilot of the present invention must be in the correct position, however, slight variation from that position will not have an impact at all on the diameter of the cold worked holes.
It should also be pointed out that although Frye et al and the present invention both contain slots in the mandrel, their purpose in each invention is different. Frye et al enters the hose coupling with his mandrel at full diameter. As he progressively expands the hose coupling, the outer diameter of the mandrel increases. The slots in the present invention are present to allow the mandrel to collapse or contract during insertion into the hole to be cold worked. As the cylindrical pilot is withdrawn into the mandrel, the mandrel expands but never beyond its original diameter "free state." As the mandrel and pilot are pulled through the hole the mandrel's outside diameter is no greater than at its "free state" or original machined dimension. Workability is obtained when compared to Frye et al since successive coldworked holes are continually cold worked at exactly the same mandrel diameter which is mandatory.
Another patent in the prior art is U.S. Pat. No. 2,508,377 issued to Doerr. Doerr shows a cylindrical pilot as does the present invention, but is not considered applicable since a mandrel isn't taught in conjunction with the pilot.